In one type of thermal printer which prints colored images, a donor contains a repeating series of spaced frames of different colored heat transferable dyes. The donor is disposed between a receiver, such as coated paper, and a print head formed of, for example, a plurality of individual heating resistors. When a particular heating resistor is energized, it produces heat and causes dye from the donor to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the donor.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel in the receiver.
The thermal time constant of the resistors are quite long so that the rate of dissipation of heat is slow. As a result, the printing speed and the image contrast are limited. Alternatively, a very high power laser can be used in thermal dye transfer printing. Laser thermal printing uses a laser as a thermal source to heat a donor containing the material to be transferred to a receiver. The donor can contain an ink or monochrome type dye material which is transferred to a receiver. When a color copy is desired, three separate and distinct donor sheets each containing a dye of a different color are used. The thermal printer registers three separate images in a receiver which, when viewed together, produces a full color image.
Current methods of producing a dye transfer image by laser energy (see UK Patent Application GB 2 083 726) consist of scanning a laser across the donor sheet and heating and transferring one dye pixel at a time to be a receiver. Because of the high energy required for thermal printing, the scanning (printing) speeds are limited.